Oral History Transcript — Felix Bloch

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Felix Bloch; May 14, 1964

ABSTRACT:Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Reinhold Baer, Karl Beck, Niels Henrik David Bohr, Max Born, Peter Josef William Debye, Paul Adrien Maurice Dirac, Albert Einstein, Enrico Fermi, Adriaan Daniel Fokker, Marcel Grossmann, Arthur de Haas, Werner Heisenberg, Walter Heitler, Friedrich Hund, Hendrick Anthony Kramers, Fritz London, Wolfgang Pauli, Max Planck, Leon Rosenfeld, Erwin Schrodinger, Karl Seiler, Arnold Sommerfeld, John Von Neumann, Pierre Weiss; Universy of Copenhagen, Universitat Gottingen, University of Leipzig, Stanford University, Teyler Foundation, and Zurich Eidgenossishe Technische Hochschule.

Transcript

Bloch:

Well, of course, I do not know. I mean if you are only interested in the history of quantum mechanics, of course by the 1930's it's essentially established, in fact has been established before. However, let's see.

Kuhn:

...Our policy as to where we cut off has simply been: Let us go to the point where the principles are clearly established and the interpretation is generally accepted, and then with each of the major applications, let us go to the point where it becomes apparent to people that this is going to work out. I mean the principles -- that now it is a matter of doing the details.

Bloch:

That, of course, was really very early.

Kuhn:

Remarkably, except in a few fields, field theory, for example; there we've tried to explore a little further with that. I'd like to know more than we do know about that transition from, in the early '30's, quantum mechanics to nuclear physics, and the application of quantum mechanical principles to the nucleus. I know, for example, and it was probably happening already when you were at Leipzig, that Heisenberg was worrying about the problem of the electron in the nucleus --

Bloch:

Oh, I see. Oh, well, he had the answer there. I mean he took me on a walk sometimes and told this.

Kuhn:

This sort of thing I do want. But...I'd be interested first in finding out something about your own biographical background. I'm interested in the new group who came into physics in these years. I'm also interested in Switzerland, because although you're not the only person who went to school in Switzerland, you're one of the very few people who was born and brought up and became a scientist in Switzerland.

Bloch:

So you want me to tell you briefly about my family? Both my parents were born in what was at that time the Austrian Monarchy. My father was born in what is now Czechoslovakia, Bohemia, the German-speaking part, and my mother was born in Vienna. For whatever interest this has, they were actually first cousins, which happened rather frequently in my family. My father came to Switzerland in the late 1890's and settled there, and my mother then came over later. I had a sister first, but she died young. She was born in 1902 and died in 1914, at twelve years. I was born in 1905, and my father became a citizen of Switzerland about that time, and I think actually -- I don't even know whether I was already a citizen when I was born, but if not, I became one through my father a year or two later. My father was a businessman, not terribly successful himself, rather more interested in educational things. I remember my mother said, and quite rightly often, that my father really would have liked best to be a teacher. He liked to read books; he was not particularly interested in sciences, but he was fascinated by numbers and so on. His real interests were in geography, languages -- he was very good in that.

Kuhn:

What had brought him to Switzerland in the first place?

Bloch:

Oh, he had an uncle; his uncle was already (situated) and was in the wholesale grain business. His uncle had moved over there and evidently took the promising young nephew into his business. Then my father decided to stay there, that's all. It's strange because the name "Bloch" is very common in Switzerland, but most of them come from the Alsace. I still have a cousin of my father who lives in Zurich, and he and his daughter -- I think they are the only Blochs that come from that part of the world. Well, I mean of course we were a Jewish family, but not of the religious kind at all. My father was not interested in religion. I actually was to some extent in my early days and still am, maybe partly as a reaction to my father's atheism. That you won't really print. I mean you'll use these words judiciously. Most of these you're not going to print anyhow...? When it comes to choice of profession, I don't really quite recall how far it goes back, but certain interests in mechanical things, at least, I must have had rather early because -- I don't remember how far back it goes, but I said that I was wanting to become an engineer. Now, in my family, they were mostly in business. My mother's brother, my uncle from my mother's side, owned a small textile factory in Vienna; so he was in that sense a bit of a technician, but also a businessman. There were some doctors in the family, but no scientists at all. Because of this inclination -- I really don't quite recall how it came -- about the nearest practical thing was engineering. And it was very appealing to me; I liked bridges and beautiful things like that. Like everybody probably, I was interested in steam engines and locomotives at that time. And so, during the whole time of my Gymnasium, high school, and I don't know -- even perhaps earlier -- I think whenever I was asked what I wanted to be, I said I was going to be an engineer. This was fine with my parents and was strongly encouraged by this uncle of mine who felt that this was really what he was missing. He thought it was a good idea nowadays to have a more scientific background, instead of purely empirical, which he had, although he was a very capable man.

Kuhn:

This is the uncle who was in the wholesale grain business?

Bloch:

No, no, who had the factory. I mean he had to deal with chemical processes, mechanical processes and so forth. Well, he thought maybe sometime I might go into the same business, but should be better prepared academically than he was. I think he went only through high school, had no university training. I did then go in the Gymnasium in Switzerland. The schools were very good in Switzerland. The elementary schools were easy, and we didn't learn too much, but they were very humanely run. We had a lot of time; I did a lot of reading and so on. I may perhaps mention one thing which I think is of some interest, and that is, the death of my sister, to whom I was very much attached, made me into an extremely lonely child. After my sister died, I was really not very much interested in the company of other children, because the grief which I had I felt I couldn't share. And consequently -- I was at that time only nine years old --. Then I was the only child. I was a doubly only child, because my sister died very suddenly, from blood poisoning, in a very short time; it was a great shock to my parents. Then, of course, they became doubly attached to me, attached and also doubly careful. I was not allowed to do many things that other boys were allowed to do because they were afraid that something might happen to me too, so I became quite a secluded person at that time, I think against my real natural temperament. And so I did a lot of reading and became a very good student; I can say that without exaggeration. In the Gymnasium, although I was good in mathematics, I couldn't say this was one of my prime interests. I liked Latin very much; I was very much interested in languages and found them very easy. I studied languages, took some music at that time like every boy of this class -- played my piano, didn't like it too much, but some. Then I became also very much interested in Nature; the Swiss mountains meant a great deal to me. And, actually I just felt, 'Well, all right, engineering seems a reasonable profession.' I liked mathematics too, like other things. My very first interest which gave me some pride -- I think in our school library, probably when I was in the first or second class of the Gymnasium, I would say maybe twelve or thirteen years old -- I got an old book on astronomy, I think Newcomb, I think it's called. I thumbed through that, and although I didn't yet know much trigonometry, I was very much fascinated; and just for myself I solved some proems like, 'How does the length of the day vary between spring and fall?' I (developed) an approximate formula and checked that it agreed, and it gave me a great deal of satisfaction. So I began sort of to feel, 'well, maybe I am really capable of doing this thing,' but still only with the idea, 'Well, apparently I will not have any difficulty.' Many boys were afraid in my generation: 'Oh, engineering, that takes so much mathematics.' That didn't scare me at all.

Kuhn:

The resistance to science and engineering was often expressed as fear of mathematics?

Bloch:

Yes, quite right.

Kuhn:

That's very prevalent today, but I have never been altogether sure where it came from or how old it was.

Bloch:

Since I wanted to go to the Technische Hochschule which was the training for engineering, I decided I would go to the Realgymnasium, which was less humanistic and which essentially meant that I didn't study any Greek, but I did have Latin. But even in that class there was a certain -- well, the humanities were the thing. My fellow pupils at that time thought that mathematics was something rather sober and dull and so forth, and also difficult. Since I didn't find it difficult, it gave me some pride, and they came to me and I helped them in solving their problems and this sort of thing.'

Kuhn:

You spoke earlier of having done a lot of reading as a child and particularly in this period of great isolation after your sister's death. What sorts of things did you then read?

Bloch:

Oh, all kinds of things, whatever entered my hands -- mostly literature -- stories, travel stories, this kind of thing. The first thing that I remember, although I probably thumbed through some books like it, is this astronomy book which fascinated me very much. Then my father took me once to an observatory at night and I looked up and saw some stars; I had the feeling that this was very wonderful and apparently people can even understand what goes on up there, although they didn't understand so much at that time. I had quite an early feeling that, 'Yes, Nature is apparently capable of rational analysis'; and also because of the great shock of the death of my sister I very soon got attracted to that because I felt, 'Well, life is uncertain, people die; here is something which is certain; this is a sound foundation.' And I think that, to some extent, was at that time a narrow but firm basis which made me very fond of mathematics. So I was very good in mathematics. Our training was excellent, I must say, at the realgymnasium. Our mathematics professor, who taught us mathematics in the last year, was a man by the name of (Karl) Beck... I remember -- that is from your point of view of interest--two teachers. One is (Karl) Beck, who worked with Pierre Weiss at that time. He was actually, although he taught mathematics, a physicist. And he re-did the Einstein-de Haas experiment -- I don't know whether you remember, de Haas got the wrong answer; he got the one that Einstein predicted, which nowadays we would say corresponds to spin one, e/2mc. Beck, under the guidance of Pierre Weiss of course, was a very good man in mathematics and found, to his great surprise, that the factor was twice as big, but he had no explanation for that. But I think the (connoisseurs) know that; that the first time this paper -- (for de Haas did it and probably) verified it. But it is true that the anomalous effect -- the suggestion came from Einstein; one could find out. De Haas did an experiment but did it poorly and found what one would expect from the point of view of classical mechanics; this man did it right. And he was an excellent teacher, so it was really a joy to take mathematics with him.

Kuhn:

Did he talk at all about this work of his own or about his work with Pierre Weiss?

Bloch:

No, no, not at all. This is something I learned later on. No, he taught us what has to be taught --

Kuhn:

The Weiss magneton did not enter into your Gymnasium curriculum?

Bloch:

No, no, not at all. I did not know as much; in fact, I knew he was a good teacher, and I rather liked him, but I didn't know how outstanding he was.

Kuhn:

Now, how far did you go with mathematics in the Gymnasium?

Bloch:

We did not have calculus. Analytic geometry was the last thing we learned, and we learned that very thoroughly, in the old-fashioned way with conic sections and this sort of thing. "At that time -- although I was not premature -- when I was almost eighteen", I did get hold of a book on calculus and got the elements, at least, of derivatives. I realized at that time that this was a very simple way of constructing tangents to curves in analytic geometry. That we weren't taught, but this I learned on my own. But that's all. We had also a very excellent teacher in physics; his name was Seiler. S-e-i-l-e-r. He wrote a book on that. Oh, there are many in Zurich who remember him. He was really very enthusiastic about his subject. I do not think that he was scientifically particularly outstanding, but he was a marvelous teacher and taught us the fundamentals very well.

Kuhn:

Now, what did that consist of, the fundamentals?

Bloch:

Mechanics, elementary mechanics, but without calculus. Mechanics, optics, a little bit of heat, some electricity but only DC currents -- no electromagnetic theory at all. And we had a laboratory; we had a laboratory in which to determine g, a little school laboratory. I think we had, except for the calculus, almost as good a training in basic physics as we teach here now in introductory courses.

Kuhn:

Were you given any sense or did you have already in the Gymnasium any sense of the crisis existing in physics, or just classical physics --?

Bloch:

Oh, well, at that time of course Einstein's theory became sort of popular, and many popular books were written about it; I read them and didn't understand them at all. I had only the vaguest idea; I was always told this was something very profound and world-shaking, and I sort of accepted it on good faith and it didn't make too much sense to me. I just didn't get the right book into my hands. Those popular books were rather badly written.

Kuhn:

But the quantum you knew nothing about?

Bloch:

The quantum I don't believe I knew much about. I don't remember where I first met the quantum. No, I really don't think in the Gymnasium. I was still of the opinion that I ought to become an engineer. I must say that towards the end, shortly before the (Matura) -- you know, that's when you finish the Gymnasium -- I began to have some hesitancy, because at that time my uncle came; I visited him often and he talked to me and he said, "Yes, that's all very fine. I'm glad to hear you're interested, but of course you must also be very sure that you get a sound commercial training to be a good businessman." At that time I began to fear that engineering might not be quite what I wanted because my family was interested in engineering from an entirely practical point of view, and this was not what attracted me very much. Nevertheless, I did enter the Technische Hochschule... I finished high school, so to say, with flying flags; I think I got the best grades. I'm sorry to say that I was not one of these one-sided geniuses, not at all. [Laughter]. I don't know whether (???) I just liked the place; it was a good school; I got along with all of my friends. So I did then enter the Technische Hochschule. I was quite interested in literature too. In fact I remember that the professor who taught us German literature was sort of (disappointed); he thought I might go into Germanistics. But this never appealed to me at all. I entered the Technische Hochschule in 1924 as an engineer and studied engineering for a year.

Kuhn:

You went to the Technische Hochschule because you wanted to be an engineer, but had you already known that you were going to be a physicist, would you also have gone to the ETH, or would you have gone to the University?

Bloch:

I've never asked myself that question. It's quite likely that I might have come to the ETH anyhow.

Kuhn:

You see, I'm curious about the relation of those two institutions. In physics the ETH has always had, or it usually had, a better reputation.

Bloch:

Yes, it did.

Kuhn:

But that's an unusual -- I mean, in Germany, you would very rarely find a major university and a neighboring technische Hochschule where the technische Hochschule was better in physics.

Bloch:

Well, of course, the University went sort of up and down. Of course, Schrodinger was at the University but was not so well known then, at that time. But I think, before that, Einstein also taught temporarily at the University. So the University (branch) went through its ups and downs. At that time, with the exception of Schrodinger who was not very well known, it was not terribly good. There was a man by the name of E. Meyer, and Bar, who was actually quite a good physicist. So the better people were at the -- there was Debye who was at the Technische Hochschule; I think I might have --

Kuhn:

He had, however...first come to Zurich to the University, before he was at Gottingen.

Bloch:

Right. And then he came back to the Technische Hochschule. So it's quite possible I would have come to the Technische Hochschule. Anyhow that choice wasn't out to me so(specifically) because I did start as an engineer. Now, fortunately the training of engineers at the Technische Hochschule, at least in the first year, was not very different from that of physicists and mathematicians. Physics and mathematics had a group together -- a little school so to say, an Abteilung -- for physics and mathematics. But I was in the engineering one. But except for some things like drawing of machines and so on, we had the normal things: calculus, which was not very well taught -- I did not have a good professor there -- calculus, projective geometry, the usual sort of things. I'm not sure whether I studied already mechanics at that time, whether I started in the beginning -- anyway, the training was not very different. But by that time, then, I became pretty sure that engineering was not my cup of tea. But to make quite sure, in the summer of 1925 -- it was recommended to engineering students that they ought to use their summer vacations to do practical work -- I did do practical work. I worked as a volunteer in an iron foundry, somewhere near Zurich; and that just about finished it. I mean it was very clear to me that this was not at all what I was looking for. It was a very empirical -- it was a small thing, of course; it had no scientific interest. It was a system, to my mind -- I was quite leftist at that time -- a system of exploitation of the workers, who were very hard-driven and so on. I wanted to have no part of it. And after that I came and I decided at that time -- well, I decided, I felt very strongly that this was not it. For a moment I thought of medicine, which my mother encouraged, but I did not mean that too seriously. Then I started to go around and ask people what they thought of my studying physics. I vent to my old Professor Seiler and he said, "Don't do it. It's a hard job; there is no meat in it. Look at me here; I'm teaching the same thing since years." I went to Hermann Weyl, who was the director of the Abteil for physics; he didn't know me at all. I reminded him later of it; he couldn't remember it, but there was just this matter, "Should I study physics?' He said, "No. You shouldn't." He was quite right. There was absolutely no prospect. If a man was that unsure that he had to ask, I think the sensible advice to him was to say "no." Well, I did it anyhow. There were a few people -- oh yes, this I might say. This was important. My father's cousin in Zurich is a lawyer. His friend from school days was a physicist by the name of (Jaffe), who was quite good; he was in Giessen. Have you heard his name?

Kuhn:

Yes.

Bloch:

He was a man who worked in the early days of relativity. He's a very old gentleman now; he's still alive. Now, my parents started to ask around and say, "What is this this boy of ours wants to go into, this futureless occupation of being a physicist?" And my uncle -- I call him my uncle; he's my father's cousin -- asked this man (Jaffe) and he had seen me or something, and had the good sense to tell my parents that they should be very glad that they had a son who knew what he wanted. And so I got a little bit of encouragement there. My father was rather sad about it, and for years didn't quite see why I chose such an abstract thing. But he was a very kind man and said, "Well, all right, if that's what you want."

Kuhn:

Do you think it was something of a dislike for the field because of its abstraction, or was it the notion of a university career, or that the teaching career vas not itself secure enough?

Bloch:

Entirely, yes. Entirely; that my father felt. Well, after all, he was not a very wealthy man; he thought I ought to make a living. The only future that I could see -- and that even was uncertain -- was to become a high-school teacher. And actually I did at the University take courses in education, because I had no qualifications for going into high school, and because it was more likely than not that that was what I was going to do... It was not easy to get these jobs. They were relatively well-paid and respected in Switzerland. I actually remember when I told it to my father, he asked me, "What do you want to do later?" and I had to say, "No, honestly I don't know." And that worried him, of course. Well, you must have heard that uniformly from this generation: the prospects were slight unless a man came from a family with scientific background.

Kuhn:

I don't know that anybody has said before that there was real uncertainty even as to the prospects of getting a good high-school teaching job -- high school in the American sense. Certainly this problem that nobody could feel any assurance of a university job was uniformly clear.

Bloch:

Yes, (the jobs were filled) very rapidly in the high schools. Jobs were not too abundant, and from what I've told you, it was people with a very high-quality education who got those jobs. It was not trivial. One really had to show that one knew something, one was proficient in teaching, and so forth. So it was entirely a shot in the dark, and I really have thought, rather proudly now, that I did that simply at that time because I thought I couldn't help it; and never mind what it leads to.

Kuhn:

Had you gotten further ideas from your first year at the ETH as to what physics was like, or was this largely based still on what had gone on in the Gymnasium?

Bloch:

No, no, of course. That's right. In the first semesters I began to realize that this was a highly active and interesting field. Although I did not know the details of the problems, I think I did know already that atoms were of interest.

Kuhn:

There do you suppose you got those ideas? What courses might you have been taking, or what lectures might you have been going to?

Bloch:

This I frankly don't recall, because physics -- then, of course, I took the elementary physics from Debye. There it was made very clear to us, but I believe I had already at that time decided to be a physicist. I think I took physics only my third semester. Well, of course, I think that probably went around. Students talked to each other and knew some physicists, and I knew some. I cannot say that with assurance. All I know is that when I got into the first contact with real mathematics and real physics, I realized this is a serious and exciting business. Although I was also worried about the material prospects, that I would feel at home in this field I had very little doubt. I may also say that I had also a little bit of success already, because our teacher in projective geometry was a man by the name of [Marcel] Grossmann, who was an early friend of Einstein. And I think, in fact, he pointed out to Einstein for the first time when Einstein struggled with general relativity, that there was this so-called Ricci calculus. I think Einstein was always grateful for him. Now he was partly paralyzed and therefore not a very good teacher, but his course in projective geometry I found very interesting. We had problem sessions. Those themselves were good people; those were not students; those were people who had finished. I remember a proof in projective geometry, which of course one gave, which I thought was very involved; yet the result was simple. I thought it over very hard over a weekend, and then sort of timidly, in the problem session, pointed out to this man that one could prove it more simply. And he was a good man; he encouraged me very much and said to his students, "See, one of your fellows here has better sense." So I got encouragement and I felt, "I can do something there." But I was still worried, because I did not know how much I could do, still. All right, so then in 1925 I entered the Abteilung fur Physik und Mathematik; in the beginning still not very much happened except this very wonderful course of Debye, the introductory course in physics. Debye had this real fine knack not only of making the basic things very clear to you but indicating, at the right moment, how they are connected with the more modern things.

Kuhn:

Was that simply a one-year series of lectures, or was it one of the standard German four-semester courses --?

Bloch:

One year. It was two semesters. It was two semesters. But every day -- I don know, four times a week or so -- with laboratory. And I believe even with problem sessions, I don't quite know, The laboratory I often neglected, I remember, and at the end I only learned what I should have known before, that a certain number of experiments was required; and I'd only done half of them. At that time I became a little --. You see, one thing was nice: because of the small material prospects, there were only a few people in this Abteilung. And they formed a sort of brotherhood, even our assistants. Consequently I was already a part of the metier. So this assistant was very nice, but Debye was rather angry that we hadn't done it, and he made us do all the experiments. So on all sides we made six experiments in one afternoon and fulfilled the requirement that way. So I was not terribly much interested in exreriments at that time. Then the first real support -- I mean Debye was a man very remote. I knew he was very famous; I began to read some of his papers. I'read his paper on the Compton effect, you know, this simple thing, collision theory, and I think I knew a little bit already about his theory of specific heat. But in the summer of 1925 I took a course -- we could take "equivalent courses" at the University -- and there was this man Bar. He gave what you would call now "Advanced Laboratory," and I was the only man who registered for that course. He was a Privatdozent then; he wasn't even a full professor. [Short interruption]

Kuhn:

You started to tell me about Bar and this course.

Bloch:

It was an advanced laboratory, and he more or less let me do whatever I wanted. I had at that time read a little bit about the Millikan experiment, so he said, "Fine, you do the Millikan experiment, the oil-drop experiment." And I did that and set up the chamber, and he gave me the gamma source and I looked at it in the dark-field microscope and came out more or less with a number for the electric charge. But the main thing was that I -- I mean what this man did to me -- he was an extremely nice man; he died very early, you know. I think he has never been quite recognized for the quality of some very fine work -- reflection of light rays from sound waves -- I think it was he who did it the first time. It was a suggestion of Debye's, but he did it. He used the standing sound waves as a sort of grating to reflect light waves. But he gave me extreme encouragement. At that time he gave me extra things to read. [Short interruption] At that time this man Bar apparently had quite a high regard for me because of my obvious interest, and I got strong encouragement from that. I also found at that time that these experiments, although they had been done before, were really interesting. And I was not decided at that time at all whether it was theory or experiment --

Kuhn:

Well, I'm curious because you said you had not liked the experiments very much in Debye's course and had done very few of them, and then you suddenly go over and you elect voluntarily to take an advanced course. Why was that?

Bloch:

Yes, yes. I did and I think it was the personal interest. Well, at that time I felt that maybe some experimental background would be good for me. When I started to take that course, I had this very great personal attention which I did not have in the big laboratory, in Debye's course. There was that difference. Besides, it was a much more interesting sort of thing to do. During that time I took Debye's course in physics, a very excellent course in mechanics, which was taught by a man [E.] Meissner,...but that's not the Meissner of the Meissner effects. He was for a long time professor. I think he was a specialist in geophysics, earthquake waves and things like that. He was not a too-fatous man, but a very good teacher.

Kuhn:

How far did that go? Did you get Hamilton-Jacobi theory?

Bloch:

No, I don't think; analytical mechanics in that sense was not taughtto us. It was rather practical; we had probably Lagrange equations, but in the applications we came to the mechanics of continua -- fluids and solids, at least in the outlines.

Kuhn:

Was there a book you used for that?

Bloch:

No. I did rather (effusive) reading at that time, because although the introductory courses were taught very well, they were not all taught. There were big gaps left. For example, in electromagnetic theory I never had a coherent course. We were left to our own resources; I read the book of Abraham on that. And thermodynamics really were never taught. Instead I read the book of (Danke). Who advised me on these books I do not know any more, but somehow I think I must have gotten good advice because these are good books. So I learned that from books. And then I also started to read the book of Sommerfeld; in fact, I think I started to read the book of Sommerfeld first, realized at that time that I would have to know a lot about electromagnetic waves, and I think then went back and read Abraham and then came back again to Sommerfeld. And of course, the reading of Sommerfeld, there I realized that this is the sort of thing which is everything, in the foreground of interest.

Kuhn:

When do you suppose you read it?

Bloch:

I would say -- I'm not quite sure -- either towards the end of my second year or in the beginning of my third year. I don't quite know. It must have been 1925 or early '26. I think, during my third year then, at the time that one began to look for something one might work in, I somehow spoke to Scherrer first. I had read something about band spectra -- it involved some calculations -- there were some people -- there was Manneback, I believe, working with Debye at that time, who had done some calculations. I became at that time acquainted with the more senior people, and I heard from him or about him and somehow thought there was something interesting to be done. I suggested it to Scherrer, and he said, "Fine. Why don't you try to measure some band spectra? We have a quartz spectrograph in the ultraviolet. Why don't you set it up?" I got this beautiful quartz prism and played around with the quartz spectrograph and started to set it up; I didn't quite know how to go about it. I went to Victor Henri [at the University] who was a physical chemist and had done some spectra, and he gave me some hints, and pointed also out to me that to adjust the spectrograph is a major enterprise, not simple. And I played around a little bit. We took some spectra with an arc source; we got a discharge, got some ultraviolet light from that and took some spectra, but they were never very good. And I became a little tired of it, and somehow drifted away from the experiment and read, as I said, Sommerfeld --

Kuhn:

Did you also at this time consider working with Debye?

Bloch:

This I want to tell you, because here now it starts becoming a little bit serious. I was still -- except with Bar, I really had no personal contact with he physicists of Zurich at that time.

Kuhn:

How about with the mathematicians? Were you taking very much mathematics along with this?

Bloch:

No, no, not terribly much. I mean I took what was required; I took a course in calculus; I took theory of complex functions with Polya; again, a very fine teacher. I took the seminar, in which, by the way, von Neumann was also .resent although he studied chemistry at that time, but he sat in that seminar. It was a very advanced seminar, together with Weyl and Polya, on complex functions. I did some studying, but I don't think that I was. Weyl and Polya knew me; I was one of the students, but I do not believe that I made any shiny mark there. There was another man, Bohnenblust, who is now a mathematician down at Cal. Tech. -- he was far superior in mathematics in my generation. Then, of course, there was Neumann, who always knew everything anyhow.

Kuhn:

I asked you this because when you begin to publish, and then particularly in your thesis, you use some fairly elaborate mathematical techniques; and I wondered where this came from.

Bloch:

I tell you... I did a good deal of reading. Once I had, I think measles -- no, no, something else -- jaundice, nothing very serious, and I had to lie in bed. During that time I read the book of Courant-Hilbert. I think that's where I really learned my mathematics. I did not only read it while I was in bed; I worked -- incidentally, I should say that, of course, was something I did already during the Gymnasium, when I was in bed, also. There was nothing else to do; I constantly did calculations, applied analytic geometry -- whatever I had learned -- to all kinds of things. This I have done for a long time. So my acquaintance with mathematics was good, but I believe what I needed for my first papers I think I essentially learned from Courant-Hilbert.

Kuhn:

You're one of the first persons who have said to me, "I not only read books, but I used to do applications, calculations in my --" That seems to me terribly important.

Bloch:

Oh, I'm sure that I owe my proficiency at a very early stage only to that. I did it as a game, just because I felt this is more entertaining than to solve crossword puzzles. In '26, or early '27 -- I don't quite know -- I had an idea. Yes, I read this paper of Debye on the Compton effect. Having read a little bit about Sommerfeld, I realized there was an assumption which Debye made in his paper. He takes an electron at rest. Then he says, "Now I come to the collision with light quantum, and I will calculate its momentum and thereby the momentum and energy of the scattered light quantum." And I said, "Well, there's something wrong here; after all, the electrons are not at rest. I just read in Sommerfeld that they are in motion." And I thought a little bit about it, just simply took the Debye formula but started with a moving electron and realized that this had some effect, at least, on the scattered light. (I went, rather proud, in) to Debye. I screwed up my courage and went to him, and he vas rather nice. I said, "I've done that." And Debye said to me, "Yes, well, that may all be quite amusing, but you know, this is not at all any more what people think about the atom. This is all old-fashioned. You should learn about the new mathematics." Now, wait a minute; I'm sorry. This happened before. I should tell you about this famous colloquium... I wish I knew exactly when that colloquium was. It must have been in (???) I believe. Do you know when Schrodinger's papers were published?

Kuhn:

Yes, '26.

Bloch:

'26. Well, it was shortly before that time. I tell you what I remember about this colloquium. Yes, I did already go to the colloquia at that time.

Kuhn:

I think from something we said in Copenhagen, there were probably two colloquia here that we need to talk about.

Bloch:

Yes. Two.

Kuhn:

One of them involved particularly the de Broglie paper. Why don't you start by telling me at what point did you start going to these colloquia?

Bloch:

I don't quite know. I might already have gone in my third or fourth semesters, but certainly in my fifth.

Kuhn:

Was this a joint colloquium for the University and the ETH?

Bloch:

Actually the colloquium was held at the University building further down there. I went there and, as I said, quite often did not understand --

Kuhn:

How big a group was generally there?

Bloch:

I would say not more than thirty.

Kuhn:

Thirty.

Bloch:

Between twenty and thirty people. This included some few students, assistants and professors, both from the ETH and the University. I don't know whether there were that many. In this first colloquium -- now wait a minute. I seem remember something else, but here I'm not quite sure -- that in one of the colloquia, Debye asked Schrodinger whether he could not report about the thesis of de Broglie. He said he would like to hear about that sometime. I believe I remember that. I certainly remember very vividly that Schrodinger did speak about the theory of de Broglie, essentially explaining in a very clear way but not adding anything new -- just this idea of de Broglie that there might be a general relationship between momentum and wave length and that the stationary orbits could be explained by simply counting off an integral number of wave lengths along the orbit.

Kuhn:

Let me ask you. Your recollection is that he did do that derivation of the Sommerfeld conditions?

Bloch:

I think so, yes.

Kuhn:

In de Broglie's paper, as a whole, that's a small point in a very much longer paper. I'm curious to know whether he really selected that out.

Bloch:

I am not sure. This may be retrospective. I would not want to swear to that. I have a feeling it was essentially a fair reproduction of de Broglie's basic ideas to the extent that when I later read de Broglie's thesis I had not too great difficulties in understanding, and that was surely due to Schrodinger. Then, and this I am quite sure of, at the end when Schrodinger was through, there were of course some questions asked. But Debye simply made the remark and said that this sounded rather naive to him; I don't know whether he used the word "kindisch" or "childish" or something like that -- "naive". He was under the impression or had learned -- I don't want to put too many embellishments in because I've told this story many times, and as one keeps on repeating it, it gets better. But the gist of it is that he felt, quite naturally, that when one talks about wave phenomena, this should be based on a discussion of the wave equation. I understand much later that this, of course, is clear because Debye came from Sommerfeld's school and, of course, they were brought up this way. And that was all.

Kuhn:

Do you have any notion when this happened?

Bloch:

No, no, I don't remember. If I wanted to find out, I would do exactly what you do now. I would find out when Schrodinger's paper was published --

Kuhn:

This was before Schrodinger's --

Bloch:

That was before, but not much before. I would say maybe three months before or so.

Kuhn:

Perhaps not. One of the stories is -- again I've got no direct information on it, but enough people know it so that I think it's true -- that at some time before the publishable papers were done, Schrodinger had a relativistic wave equation; and that he put this away because the results came out badly, and it was only later that he came back and did the non-relativistic one.

Bloch:

As far as I remember, the time-dependent Schrodinger equation was not terribly important at that time. I think he used the time-independent Schrodinger equation to get the energy levels.

Kuhn:

In fact in the first three papers he's got a time-dependent Schrodinger equation with a second time-derivative, which he only later changes.

Bloch:

Yes, but I mean his real results he got from the time-independent because he wanted to get eigenfunctions and eigenvalues. To my recollection it cannot have been more than a few weeks after this de Broglie seminar -- I think it was only two weeks later, but it might have been three -- that Schrodinger said that Debye had suggested to him this idea of a Schrodinger equation and he had it, a wave equation. Don't ask me too many details because I was a very young student at that time.. It is quite possible that he started with a wave equation of second order like one does in optics --

Kuhn:

He lectured on this, you think, shortly after -- on the wave equation. Did he produce some applications?

Bloch:

I think, but again I'm not quite sure; I think he had already the energy level of the hydrogen atom. And there I know he got a very great contribution from Weyl, because he didn't know too much about eigenvalue problems of differential equations. Now I cannot quite swear to that, but I was certainly impressed at that time by the shortness of time interval between the first suggestion of Debye and the fact that Schrodinger had an equation and had results.

Kuhn:

That's terribly interesting. It puzzles me because, as I understand it, there is an intervening stage in which he has a relativistic equation and results that are not coming out right. He's got the relativistic equation with no spin terms.

Bloch:

I don't recall that phase. If it was there, I'm sure it was of very short duration.

Kuhn:

That may well be.

Bloch:

Because it did not take him long to get the right equation. He had realized then at a very early stage the connection between the wave equation and the Hamilton-Jacobi equation, which is of course first-order in time. I think he latched on that quite early.

Kuhn:

Do you remember at all how he derived his wave equation when he first showed it to you? Because in the published papers it goes through various stages.

Bloch:

No, no, I do not remember that. I do not remember.

Kuhn:

There is another story about a remark of Debye's at about this same time. I wonder whether you have any recollection of that, which was that Debye, after first seeing the way Schrodinger handled the wave equation, said something to him about it--that "if it's a wave equation, you ought to be able to derive it from a variational principle."

Bloch:

That I do not recall, no. That I do not recall.

Kuhn:

How did you, and how did Debye, and how did others feel about the Schrodinger equation?

Bloch:

I do not believe that I understood at that time its importance. And I'm not quite sure that Debye did. Debye probably felt that it ias rather trivial. That there was something exciting and new coming on I think was not clear to me until about the summer of '27. The first thing I remember there was the question about the significance of the wave function. Schrodinger's ideas that the Ψ2; He did not like this idea, never liked the statistical interpretation. He thought that the wave packet which one could form represented the actual particle and its shape. I think it goes back to that time that I began to realize that something very fundamental was at stake -- not before that.

Kuhn:

Had Schrodinger lectured on this outside of the colloquium? Had you seen more of him after this --?

Bloch:

Yes, I had. This is what I wanted to say, and I think here my chronology is not quite right. There was this colloquium which I told you about--these two colloquia. I believe my visit to Debye concerning the Compton effect was either shortly before or shortly afterwards -- probably shortly afterwards -- because when I came to him with this idea, he said to me, "Don't bother about this old-fashioned kind of atomic mechanics. Learn the new quantum mechanics." And this of course I did learn; I learned it to a very large extent by reading Schrodinger's papers.

Kuhn:

Let me simply raise a point with you. If it is before the colloquium then he can scarcely have meant --

Bloch:

That is not so certain, because of course there was Heisenberg's.

Kuhn:

Yes. what I wondered, you see.

Bloch:

However, I do believe -- as I say, I'm not quite sure -- but I do believe that when Debye made this remark to me, he did not mean for me to study matrix mechanics, but he meant for me to study wave mechanics, which is in fact what I did.

Kuhn:

Were you involved with Heisenberg's mechanics, at any point?

Bloch:

No. Later on. Later on, but not at that time. No, I think I didn't take that seriously up until when I came to Leipzig. So as I say, I'm unfortunately not quite sure where to place this incident with the Compton effect. I rather think now it must have been after. I think wave mechanics was already in the coming and Debye was sufficiently convinced that that was the right way to do it and that these quantized orbits were out . He gave me this very excellent advice that I should study that. Then I did get into contact with Schrodinger -- personal contact. Also what happened very much at that time was that Heitler and London came; this was in '26. They came in '26, and they were very nice. Although I was only a young student I established at that time a very close friendship, and they took me on walks, and sort of told me they were working at that time on the covalent-bond theory. I think I began already to understand what it was about, and they helped me, explained it. So I got quite a bit of stimulus from them during the time when --. Of course, during that time I studied -- this must have been in early '27. I read the papers of de Broglie and of Schrodinger and began to understand, really, wave mechanics and the Schrodinger equation. Then in the spring of 1927 I cane to Schrodinger. I had heard, either through him or from London or from Heitler, about this difficulty with the wave packets; and I had an idea which I thought was very good.

Kuhn:

When you say "this difficulty," tell me what you mean.

Bloch:

The interpretation of the Ψ2. And the fact that wave packets ran which bothered Schrodinger very much. So I thought I could help him. went to Schrodinger and I said to him, "Look," -- of course, you will be amazed how naive I was -- "so far people have always spoken about the motion of electrons in an atom, but they haven't taken radiation damping into account. Maybe radiation damping will hold all the waves together." And believe it or not, Schrodinger said, "That's a very good idea. Why don't you try it?" It was a very lousy idea. That dialogue with him was the reason why I took up the problem of radiation damping. Actually, I did not yet know Dirac's --. I'm not quite sure, because part of this first paper on radiation damping I had already before I came to Heisenberg. I must have treated radiation damping in a somewhat more globally. I do remember that the main result came out right. I did it for a harmonic oscillator, and I could show why the harmonic oscillator had the nice property of staying together anyway. With damping, it not only stayed together, but the amplitude actually died out. This I was rather proud of. Beyond that I didn't go. Then came a great crisis in Zurich because at the same time Schrodinger, Debye, and Weyl all left.

Kuhn:

I didn't realize that Weyl had left at the same time.

Bloch:

Yes. He also. Weyl went to Gottingen, Schrodinger went to Berlin, and. Debye went to Leipzig. And then it was clear to me -- I was about to start my thesis, and everybody with whom I would have cared to work left. I talked to Debye about this problem, and he was very nice. Oh yes, incidentally, some time during that time Scherrer once came to me and said, "What's happening with your experiment down in the basement? I haven't seen any changes for several weeks." And. I said, "Yes. Well, I now study quantum mechanics." He said, "That's fine, as long as you do anything at all." And that was the end of my experimental career, at least temporarily. I spoke to Debye at the time, and he decided to go to Leipzig, which was probably the spring of '27. He suggested to me that I should go to Leipzig and study with Heisenberg. I don't quite know why I took his advice, of course, but I had heard about Heisenberg; I respected Debye very much. Maybe I also thought I'd better go to a place where I knew at least already one of the professors. Anyhow, so then I went in the fall of '27 to Leipzig...

Kuhn:

Before we begin with Leipzig I should like to learn more about how you and others felt about the interpretation problem and the wave-packet problem while you were still in Zurich with Schrodinger.

Bloch:

I was too naive to appreciate the depth of the problem. Probably more or less on authority, namely on Schrodinger's authority, I also thought that there was apparently something missing in the theory because of the very fact that this ought to stay together -- an electron doesn't become a mile long. And so something must have been forgotten. That there is a statistical interpretation I do not think was pointed out to me very clearly. I'm not quite sure to what extent even London and. Heitler knew about it or were convinced of it. Have you spoken to Heitler about that?

Kuhn:

His memory was fairly vague on this.

Bloch:

On this point of the interpretation of the wave.

Kuhn:

Yes. He had been at Gottingen. I think the chances are good that he was already working on own interpretation. But I'm not sure.

Bloch:

I do not remember that in my discussion with Heitler and London this played a great role. And therefore I do not believe that it stood in their foreground of interest particularly. They probably didn't even care very much about it because all they were interested in at that time were the energy levels, and they were very much intrigued about these two symmetry classes, one leading to attraction the other to repulsion -- they didn't need really all this. And if they thought it was an important problem, they certainly didn't bring it to my attention. I must say I was not aware of the problem, except that I realized -- this much I understood -- that wave packets have a tendency of running apart, and then I had the idea maybe I could keep them together.

Kuhn:

Were you worried or were people in Zurich worried about some of the other problems that get emphasized in this respect? For example, the fact that transition probabilities should be proportional in the Schrodinger theory to the intensity in both the initial and the final state? You know, he viewed r ation as a beat phenomenon between two levels.

Bloch:

Yes. I did not speak to people about that. It seemed to me at that time very natural, and I did not meditate on that. I just simply said, "Well, that is a rule, and it works." I was not mature enough -- I was only twenty-one at that time -- I was not mature enough simply to appreciate the depth of the problem. I was pleased. I said, "Well, fine. There is a new mathematics --"

Kuhn:

Do you really think that's a matter of maturity? I don't mean now that maturity is not also involved, but isn't it also true that there are different sorts of physicists? That some would say, "Here is a rule that works, and I will not --." Wasn't Debye a very good example of a man who very often brushed aside the deeper problems and said there were no problems there at all?

Bloch:

Quite, quite.

Kuhn:

He was also a very mature scientist.

Bloch:

Well, mature, perhaps so. Yes, all right. Not philosophically minded., let me put it that way. My attitude was, of course, [determined] through the influence of Debye very much at that time. I mean nobody pushed me into these philosophical problems; I didn't care. I felt, "Well, there are nice things coming out, and let's see what's coming out of it." That there is something profound in it I had a first inkling of, but only an inkling, under Heisenberg, and to the real things my eyes didn't become opened except through Bohr. That was much later. In fact I had some reluctance at the beginning, a typical Debye attitude. by speculate about these things we know how to calculate? Only much later I realized that. So at that time there was talk, I think, in Zurich that Schrodinger had some idea and people didn't ree with them, and it seemed more likely that Ψ2 should be a probability. I don't know, I must have shrugged my shoulders to that, or less. So I was really, shall we say, rather indifferent... [short interruption] ... to the interpretation of the wave function. Coincidentally, I think I know quite well now how I did this radiation problem that time, because I think all I did, since Schrodinger had already calculated transition probabilities, was simply to say, 'All right, there are certain transition probabilities which go from one state to the other.' And I simply took these into account as damping in the wave functions and then built them up to wave packets and noticed that the whole wave packet is damped. That's how it was.

Kuhn:

You went off at this point, then, to Leipzig?

Bloch:

Yes, that's right. In the fall of 1927 I went to Leipzig. I had something written up about that. In fact, I had reported at some minor meeting, I think, in Zurich or sel or some place. When I came to Leipzig Heisenberg wasn't there yet. I knew he was about to come. Only Wentzel was there. He was extremely kind. I showed him that paper, and he looked at it and he said, well, he didn't feel very competent about these matters, and he suggested that I should show it to Heisenberg. Beyond that, we didn't really go very far. It was only a week or two before Heisenberg came back. Then I think Wentzel must already have mentioned it to Heisenberg or -- I don't quite know. Anyhow, when I saw Heisenberg, I told him about it. He listened to me, and he immediately discarded it. I don't know, I think I realized already at that time that this isn't going to do anything to wave packets. I mean, he discarded that; he said, "No, no. That's not so." He made me feel, "This is quite clear. This is a probability, and don't worry about it any more." And in my unphilosophical attitude I accepted that. However, he did say that the problem would interest him, that it should be done more thoroughly, more generally, not only for the harmonic oscillator, but generally speaking. I don't know whether he said --. Yes, I suppose he said to me that I should really use Dirac's radiation theory, which was developed at that time. I think I knew already what he was talking about; anyway I studied Dirac's paper and then published this first paper on radiation damping [paper No. 2]. This is more general -- in fact, it showed something which pleased Heisenberg very much, namely that if radiation damping does anything -- of course, it doesn't do much anyway -- it tends to even destroy phase relations. That is to say, the wave packets run even slightly more apart. And that was that. I can say that was finished.

Kuhn:

... Coming to Leipzig from Zurich, and coming to Heisenberg in particular, you may have gotten suddenly quite a different impression of what was going on in the field. Did this change things for you a lot?

Bloch:

No, I have a feeling that the transition was rather gradual. You talk again about the interpretation?

Kuhn:

Let's say clearly the interpretation would be one of the issues, but I wonder what Heisenberg's attitude was toward the whole Schrodinger approach still? This is still '27.

Bloch:

Oh, very positive. Schrodinger [sic i.e. Heisenberg] had already himself used the Schrodinger equation to explain the ortho-para hydrogen. No, no. I mean, as a technique he thought it was fine. Heisenberg did by no means, for example, tell me at that time, "Well, you'd better study some matrix mechanics." Heisenberg was not a man who cared about methods. He felt that the Schrodinger equation was a very convenient method. Incidentally, of course, through Schrodinger's paper in which he established the connection -- which we had read by that time -- with matrix mechanics, I knew enough about matrix mechanics. Heisenberg fully realized at that time, as I think everybody did, that the two methods were equivalent. And Heisenberg was not a man of narrow prejudices, I mean. Whatever worked, was fine. So he was very pleased that I did know something about the Schrodinger equation, and that that was equivalent to the matrix mechanics I knew. The interpretation, as I said, didn't bother me very much, except that Heisenberg told me simply, in a very quiet way, "No, of course this is not going to do anything, but it is an interesting problem." I immediately realized, without any regret, that this idea of the holding over of wave packets would go, and I evidently felt, "Well, the great scholars don't see any particular need for it, but there is some point at least in going into radiation damping, because it hasn't been done before." I should say I had a tremendous impression of Heisenberg, right from the beginning. I mean we struck it off very well; he was extremely kind to me. I was his first student so I had a lot of time; I participated immediately in the seminars and established a very close and good connection with Heisenberg and admired him tremendously. Consequently, a statement like that I would of course take. It was of course true, and I had the feeling, "Well, that settles it. There's no use worrying about that any longer." Heisenberg himself was not too much interested at that time in questions of epistemology. I think he was quite happy about his gamma-ray microscope, and that answered the question as far as he was concerned. He [had] also a little bit of this sort of aggressive way, like saying, "Well, now we have a new --. Let's try to understand metals and this and that."

Kuhn:

When he got there, he had recently been both at Como and then at Brussels at the Solvay Conference, where these had been very live issues. Was there any carry-over that you remember --?

Bloch:

Not that he transmitted to me, no.

Kuhn:

Would you say that it was not only you and he, but that in practice at Leipzig, even that early, these interpretation things were already pretty much dead issues?

Bloch:

I think so. Yes, yes. There was no great discussion. Really, I must say that there were problems and deep problems I wasn't even aware of, except through rumours occasionally, until I came to Copenhagen.

Kuhn:

That fits much with the impression that I had had. Until one's heard it three times, one is never sure, in view of the nature of memory, that it's going to come out the same way.

Bloch:

I must also say, although at that time I began to know several of the younger generation -- let's see, soon afterwards Teller came, and Weizsäcker was there, and I mentioned already Heitler and London -- none of these people, at least at that time, seemed to be particularly excited. by the interpretation. This was the Copenhagen spirit that had not yet particularly penetrated. I don't know, I may be unjust, but I have the feeling that even Heisenberg felt, well, it was an interesting question but not all that interesting. Heisenberg was himself quite young at that time and not really philosophically minded. I think he was quite --. He was really more interested in seeing what consequences one could draw from the theory than to --. I mean, for his taste, the ΔpΔq settled the business.

Kuhn:

What problems was he working on? How did you get to the theory of metals?

Bloch:

Now, let me see. Well, that was his suggestion. He was interested in sort of down-to-earth applications. After I had finished this paper and written it -- that was in the winter or late fall of '27 -- I went home; and he said to me at that time, "Well, I think you ought to think now about a thesis problem. Why don't you think about some questions, what you might do?" I came back with some rather poor suggestions; I was not very educated at that time. I thought, for example, that it might be of interest to follow Ehrenfest's adiabatic theorem through, see how it goes in quantum mechanics. That question apparently was so clear to Heisenberg that he said, "No, I don't think that's very interesting."

Kuhn:

Wentzel did that too, didn't he?

Bloch:

I don't remember. Not at that time. Heisenberg felt that it was a purely formal problem because he could quite clearly then see how in an adiabatic transformation the states in quantum mechanics go into one another. Well, he suggested two problems that show very clearly what his interest was. One was: Sommerfeld had written before papers on the conduction electrons and had pointed out that the Fermi statistics give the right behavior in experiments on specific heat and some of the thermoelectric effects, and so it was clear that there was more to be done about that. Heisenberg said to me, sort of in a gentle way, "Well, maybe one should see whether the electron theory of metals, what that becomes like in quantum mechanics." That was one possibility. And the other he mentioned to me was the idea of ferromagnetism. I knew very little about magnetism at that time, but he mentioned to me --. He said, "Well, there is that Weiss field, and it strikes me as very similar to the problem of ortho and para helium." That is to say that the para position has really to do with the symmetry and therefore it's an electric force. He sketched out to me briefly why that field is so very great because it's not really of magnetic origin. So that was the gist of this problem. But I think I felt, "Well, Heisenberg has it already in a nutshell." That was his first idea. And then I said, "I don't want to just simply work it out." Besides, I didn't probably realize at that time that this was a problem of major importance: 'Well, very nice. So OK. One can't explain the Weiss field, so what.' So Heisenberg did that himself. But I was then interested in this question of metal electrons, and very soon -- in fact, I think only a week or two after, he told me that -- in some primitive way I had gotten the essential idea that a periodic arrangement is not really an obstacle for waves, but it's only the thermal vibrations. Heisenberg was very pleased. I told it to him only in the one-dimensional case, in a very primitive way. And he said, "That is explained now. Now I understand."

Kuhn:

Did the problem you set yourself or that Heisenberg set for you include the notion of getting the potential back in? I mean this Sommerfeld had not done.

Bloch:

No, no. No such thing. It's just that, in a general way -- after all, Sommerfeld had only applied the statistics -- but there is more to it. Maybe it's conduction; maybe it's a question of metallic resistance. Itought to be treated properly. And that's clearly it. More than that he didn't say.

Kuhn:

In the first of the published papers you bring in group theory very quickly.

Bloch:

I did that. That was the fashion. That was my tribute to fashion. I did not want --

Kuhn:

-- I was going to say, you helped to establish that fashion!

Bloch:

No.

Kuhn:

I don't mean that you invented group theory --

Bloch:

No, no, but the application of group theory came from me.

Kuhn:

Bloch:

Well, the most trivial of all groups, you see.

Kuhn:

But, still and all I would say that at this time an awful lot of people are still saying, don't have to learn that sort of mathematics." Heisenberg wasn't using group theory that early. He's also not failing to solve problems because he isn't using it.

Bloch:

Well, I mean it was talked around at that time that group theory is very profound -- a lot of mysticism was made about it.

Kuhn:

Now, already --? Where was this going on?

Bloch:

Oh, I should say this: we had a very lively communication with Berlin. We were quite frequently at colloquia in Berlin. The whole group -- Heisenberg and Wentzel and the students -- went up to Berlin. So, no wonder! That was probably there somewhere in the Berlin atmosphere. Von Neumann was there; we knew him well from Zurich, and he mentioned it to me and talked to me about that.

Kuhn:

Had you really gotten to know him fairly well in Zurich?

Bloch:

Not too well, Well, you know, a fellow sits at the next bench; I knew he was a genius; that was quite well known.

Kuhn:

Was that generally known already of Neumann right from the beginning?

Bloch:

Among the "ins" it was known. Weyl had a very tremendous respect for him, and I could see in this advanced, seminar when Weyl didn't know the answer he would say, "Neumann, how does that go?" We all realized this was a great mathematician. Then I met him again in Berlin. He was extremely kind, and I was at his house; and I think through conversations with him -- I knew Neumann more than Wigner at that time. I had the impression that group theory is something tremendously important. Later on, I didn't think so much of it any more, but at that time I did. And the idea of why a periodic potential doesn't really present an obstacle to a wave --. I mean I didn't know anything. Of course, if I'd known about periodic potentials -- the work of this astronomer -- I would have known. But, anyhow, I did it in an extremely primitive way. All I did was that I said, "Well, I'll make a Fourier analysis; if it is periodic, then I'll make a Fourier analysis of the waves also." Then I realized that nothing is happening; in fact by putting the Fourier components together, I realized that it's a wave which is modulated. That's what I told Heisenberg, just in that primitive way, for a one-dimensional lattice.

Kuhn:

Aha! So the whole group theory comes in later as a way of doing this --

Bloch:

That's a tribute to the fashion. It was simply that I felt at that time that whatever you can do with group theory you ought to do. It is not the way I started into it. And. I think it was my first and last application of group theory. [Chuckle].

Kuhn:

It's a neat one. It's a very nice, general proof.

Bloch:

Anyway, that's not the way I did it. And so this was the first thing. Then Heisenberg felt -- well, he was very pleased and said that was clearly the key to the whole question of what one --. And I may say this much: that although Heisenberg was a tremendous stimulus, gave the problem himself to me and was always very encouraging, the way how to go at it I think was really my own. That one first had to understand what happened in this strictly, purely periodic potential was clear to me. It was clear that Sommerfeld simply considered it as if there were no ions at all, and I knew, of course, there were these ions. So that was it. From then on I went pretty much on my own, with occasional help and correction from Heisenberg. I would say I saw Heisenberg regularly. Well, I saw him often in the seminars and so on, but also occasionally he said, how is it going?" Sometimes I had difficulties, and. Heisenberg,made a suggestion to me: "why don't you try it this way?" And, very often that was it . So I wrote this paper on the conduction of electrons in a very short time; I think that in about half a year it was finished. I made a mistake there, by the way. You don't know these things so well. I came out, for high temperature, with the right law, linear with temperature. But for the low temperature the law was wrong; I had a T3 law. This was a sloppy discussion of an integral equation which Heisenberg didn't catch because Heisenberg wasn't much of a mathematician, ever. [Chuckling]. He believed that to be --. He shouldn't have believed it. I did that the next year, a year later, maybe two years later. Then I corrected it; that was T5 there.

Kuhn:

Did you go over it yourself and find, the mistake yourself, or did somebody call it to your attention?

Bloch:

That I don't quite recall. That I don't quite recall. I had the feeling, probably, at that time, that I'd pulled some swindle there. I wasn't very sure of it. It was one of those tricky cases where, in the limit --. Because, neglecting certain terms, I got an integral equation of which the homogeneous equation had, a solution, I sort of glossed over that. Later on it occurred to me. I knew one had to be very careful in this case. Then I realized, 'If I keep certain terms, it doesn't have a solution. At that point they must be essential.' They were small terms, but they were important because they made the difference whether the determinant is zero or finite. That sort of thing. I think I had a bad conscience at that time. Actually, now, I don't think anybody put me [on] to it. But 'then shortly afterwards I met Grüneisen. I'm going ahead of my time; that comes later. This was then my first year in Leipzig. I don't believe -- except that I learned an awful lot from Heisenberg, had talks with him. He did, of course, at that time work out a theory of ferromagnetism. I learned it directly, first time, from him. He began to work already with Pauli on the quantum electrodynamics and told me occasionally about the work. He was extremely optimistic. He felt, well, of course, now one has to quantize the electromagnetic field. He didn't foresee any difficulty there. He just said, "Well, everything will come in order now. Clearly one must just carry this program which Dirac has really started, must carry it through; and everything will will come out fine." I do recall that he came to me occasionally because he was in constant correspondence with Pauli. What usually happened is that Pauli pointed out certain difficulties to him, and then Heisenberg had solutions. For example, this business with divergence of is equal to lap. But this is really a boundary condition, you know. And. Pauli pointed out that difficulty to him. Then Heisenberg had some way, a rather 'dirty' way I think, of getting over that difficulty. I swear they somehow, by-passed it. They didn't have the neat way that Fermi later had. I just remember that much of the time, and. I do remember the extreme optimism of Heisenberg. He thought that all the difficulties were going to disappear. How soon he realized --. I think that was later, when I was already gone from Leipzig.

Kuhn:

Was anybody else in Leipzig at that point also interested in the field quantization problem?

Bloch:

I don't think so. There were very few people in Leipzig. During that first year, I'm not quite sure --. Peierls was already there, yes; but I think he was only a student. He was not sufficiently advanced.

Kuhn:

You didn't get involved at all with field quantization, did you?

Bloch:

No, no.

Kuhn:

Did, you follow even the papers that were coming out then?

Bloch:

I think so, yes. I think so. I did a lot of reading. I read Dirac's paper at that time.

Kuhn:

What about the Jordan-Klein and the Jordan-Wigner papers?

Bloch:

... I knew about them, but I don't believe I used them, nor did. I probably at that time appreciate their importance. I mean, I felt, well, that's very nice; instead of working in the space of many dimensions, you can do it this way. It sounded rather complicated to me and didn't seem to offer anything new. In fact, it didn't, you know. It's not really until Fermi used these as creation and destruction operators that the real depth of it came out. It seemed to me a formal device... I don't believe that Heisenberg was terribly much interested. Well, he knew about it, and he said, "Yes, one can do it that way." Transformation theory I think I studied at that time; that rather interested me. Yes, that I knew. I have a feeling that at that time I paid really more attention to Dirac's papers than to the Gottingen school.

Kuhn:

You were there when the Dirac electron came out, weren't you?

Bloch:

Yes, yes.

Kuhn:

What sort of an impression did that one make?

Bloch:

That I can tell you. This was very interesting. Heisenberg told me about it and said, "Dirac had the relativistic Schrodinger equation where the spin came out." And then I read it. I came afterwards to him and I said to him, "It sounded rather obvious to me." Heisenberg said, "Yes, one thinks that always afterwards." There I had the feeling that was a naive, cocky approach; and Heisenberg told me quite plainly, "Well, this is more an achievement than you realize." I was not very much impressed. I just said, all right, so what? I knew that a simultaneous system of several first-order equations gives you a higher-order equation.

Kuhn:

You're the first person I've ever met who reacted to that that way.

Bloch:

Maybe I'm the first honest person --. I thought it was trivial. Heisenberg pointed out to me that there was more to it than meets the eye.

Kuhn:

Was it trivial that, going at this from the point of view of a relativistically invariant wave equation, one should come out with the spin matrices without ever putting in anything about spin?

Bloch:

It didn't impress me very much, I must say. The whole reasoning of Dirac sounded so misleadingly obvious when he said, "After all, the Schrodinger equation must be first-order in time." Then he said, "How do we get [that] out? Naturally, by simultaneous equations. But I can't do it with two, so I need four." That was too bad, but all right. That the spin came out -- No, I think that was really not too surprising, because that the spin was a relativistic phenomenon was, of course, well-known. And, that came probably from Sommerfeld via Heisenberg to me. After all, Sommerfeld had had a derivation of the fine structure as a relativistic effect. The effect of the spin was just of that nature. And that it was a relativistic effect was clear. Well, as I say, my reaction to that was evidently quite unjustified, but I do remember that I didn't have the slightest doubt it was right. As I say, I thought that was a rather simple --. And Heisenberg corrected me on that. It was really a year of tremendous growth because really I think that at the end of my Leipzig period, when I got my Ph.D., though there were still very great gaps, I think I was essentially at that time a physicist. I think I knew the more important things which were going on in physics, except for the epistemological things, which I just paid no attention to. I had also a transitory period quite near the beginning, where I was interested in this five-dimensional theory of Klein and. Kaluza. That attracted me for a while, and. Heisenberg said, all right --

Kuhn:

In this year?

Bloch:

Just after I came, after the radiation paper or during the radiation paper, I participated in a seminar; and, since I was interested in these things, Heisenberg suggested once that maybe I would give a talk about it, which I did. But Heisenberg was not very much interested in this sort of thing; he felt it was a rather formal approach and directed me -- and I was very willing to be directed in that direction -- toward more down-to-earth things. He didn't want me to get lost in this.

Kuhn:

No. Where had you gotten the relativity theory for tha

Bloch:

That I studied on my own, as a student. Oh, that I can tell you. There my source of information was Pauli's Encyclopedia article; that I read in my second or third year. And again -- I read it, but read it with a pencil and paper. I must say I really know my relativity from there, and. I. know it well.... Well, I think I'm essentially through with my Leipzig period. I had three periods in Leipzig. And I'm telling you about the first.

Kuhn:

Three I didn't know. I knew only the two. This first one was simply '27- '28?

Bloch:

Right. Then I went to Pauli. I can just tell you in a nutshell... '28- '29 I was with Pauli; '29-'30 I was in Holland with Kramers. Then I came back to Leipzig, but as an assistant, and was in Leipzig 1930-31.

Kuhn:

The full year?

Bloch:

The full year, -- or was it? -- yes, full year. Then I went to Copenhagen, for half a year, till the spring of '32. And then I was one more year in Leipzig, until the spring of '33. This is it.

Kuhn:

And then from there to Rome?

Bloch:

Then I went to Rome. But then I spent a summer, after I left Leipzig in the spring of '33, left Germany, I spent a summer partly in Zurich at home and partly traveling around visiting again. I was in Paris and visited again Kramers. Then I went to Rome for half a year, and then I came here. hat's it. You want to ask about this first Leipzig period or about later ones?

Kuhn:

I have no other questions to ask about the very first Leipzig period, unless you have other things that you remember about it. The questions that were on my mind, that I had to ask, you have answered. I would say, though, why don't we stay chronological? We will come back there when you get back there.

Bloch:

OK. So first I went --. Well, I mean Heisenberg recommended me to Pauli. I was an assistant. I must say that I went to Zurich with some misgivings; I was afraid of Pauli. I'd met him before, and I knew his sharp, critical tongue; and I was afraid, and Heisenberg thought that was --. He laughed it off.

Kuhn:

Were you his first assistant at Zurich?

Bloch:

No. Kronig was. Kronig had been there before me . And. Kronig, I think, had gone back to Holland. Anyway, I was his second assistant. Kronig, I think, had, been there half a year. And I came then in the fall of '28; I was his assistant. I didn't have much to do as an assistant. I corrected some papers of his students. Research --. He said, "Yes, well of course the normal theory of conductivity, that's fine; but the real interesting problem in this field is superconductivity. Why don't you work on that?" And I did work on that, but unsuccessfully, for a long time. It was rather interesting because I --. First, I may say one thing. There was an idea about superconductivity which Landau and I had independently. And, that was the basic idea which Pauli immediately accepted: namely, this immense stability of superconductive currents cannot be just a selection principle, some very highly forbidden transition, because nothing in the -- world is that highly forbidden. It must be -- and I compared it because I knew the theory of ferroma etism of Heisenberg -- compared to magnetism. Landau and. I independently. Because the reason, of course, why you have spontaneous magnetization is that this is the lowest energy state. And so for the same reason we felt that there must be an energy minimum, connected with a current. The strange thing is that in a superconductor an energy minimum is a current. Now, this is the basic idea Pauli accepted, and he said, "Yes, of course. That's the explanation," which I think it still basically is. And it sounded rather simple to say, "All right, now you just have to take interaction into account, and it'll all come." I did that; it was all wrong. And I produced theories at the rate of about one every few weeks, and it took Pauli usually between five and ten minutes to see the flaw in it and send me home. Pauli got rather angry at me, and annoyed: "This is a simple problem. Why don't you --. You're always making these mistakes!" he said to me.

Kuhn:

What sort of mistakes would he find, in the mathematics?

Bloch:

Yes, yes.

Kuhn:

Places where you had persuaded yourself that you would get a current term that he --?

Bloch:

Yes, exactly. And then he said, "Well, that I don't believe. You have neglected this term; I'm sure you can take it into account. I'm not convinced." Or something like that. He was right, you see. And so all the models which I tried were much too simple-minded. That is to say, after I'd done them right, after Pauli had corrected my mistakes, it always came out that at an energy minimum there is also zero current. Very simple-minded models. Once Pauli was apparently not awake, or he felt too well, and he believed it, what I had. So naturally I believed it too, and. I started already to work on the magnetic field. I remember he talked to Otto Stern who was there at that time, and said, "Yes, I think that Bloch knows now the theory of superconductivity. There are some more details about the megnetic field he's worrying about now, but that's not important." They didn't catch me that time. But later on someone caught me, maybe he again. It was also wrong. So I got nowhere. I wrote only one paper at that time. It was a rather unhappy time because Pauli was very critical, and it sort of depressed me. I wrote only one paper at that time, which was of some slight importance because I felt that the model which Heisenberg -- I began then to be interested in ferro-magnetism -- that the model which Heisenberg had used for ferromagnetism was not necessarily so. On the other hand, one had this model of metals, of conduction electrons; and I wrote a paper, which I think you have also, on the possibility of ferromagnetism by conduction electrons. This one, yes. 1929, that's right [paper No. 6]. And this [indicating paper No. 5] was a paper that was more or less -- yes, this I forgot -- an outgrowth of Pauli's paper. I think much of Pauli's contributions I learned at that time, but Pauli had written -- well, the so-called temperature-independent paramagnetism. And this was closely related to it, Pauli thought, (???) Pauli, and so it was. So these are the only papers I did write in Zurich.

Kuhn:

You had been in on the Heisenberg end of this correspondence with Pauli about quantum electrodynamics. You must now have seen Pauli's attitude. Did you find that very different?

Bloch:

Yes, yes, very different. Pauli was much more critical about the whole thing. Pauli was in depressive moods at that time anyway, which he communicated to me. He did not feel that things were going well at all. That is to say, he had the feeling that these were deep difficulties. I don't believe that I understood the difficulties. Well, I already realized that there were these divergences which were simply not to be overcome, or not --. I don't quite know. I rather suspect that Pauli knew that, and did not share Heisenberg's optimism. Of course, I do not know whether at that time Heisenberg was still that optimistic. But it was really for me --. The transition from Leipzig to Zurich was to go from optimism to pessimism. And that has partly to do with the character of the people, but I think it has also partly to do with the development of physics at that time, because the early optimism which reigned had not been so (???) began to look rather skeptical. There was one very interesting remark that Pauli made to me at that time, which I remember and therefore it must have impressed me somewhat, although I didn't do anything about it. He was very critical about Dirac; he always was. He thought that Dirac was a rather shallow thinker, and of course only a man like Pauli has a right to say that. And he didn't believe at all that Dirac's reasoning that the probability density has to be positive was right reasoning. He said to me, "Well, why does he speak about probability density? Why doesn't he speak about charge density? I do not see why charge density couldn't be negative. It can be positive and negative." In other words, Pauli quite clearly realized already at that time what later was to become Weisskopf-Pauli.

Kuhn:

That's very, very interesting.

Bloch:

I had thought at that time, as I told you, that Dirac's paper was trivial, because the reasons seemed to be so straight-forward. And Pauli pointed out to me, just as a side remark, that far from being trivial, it was not at all convincing to him. It's not that he doubted the Dirac equation, but he just didn't see the necessity of it. It's quite clear that he had already the basic ideas of a theory which was positive-definite. This is always what he said. He said, "This is not a good theory where you have negative energy values. And I think what one should have --." I said, "Yes, of course that's true, but if you want to have the density positive --." He said, "Why should the density be positive? I'd rather have the energies positive, and let the density have both signs." You see. And that's of course what he achieved in the Weisskopf-Pauli theory.

Kuhn:

I had no notion that it went back that far.

Bloch:

I may just tell you -- this is a little bit anticipating, but it was quite clear that Pauli had all the basic ideas. Now it is a little bit of a nasty remark, but I think that since you are historians, we have to say it. When I once spoke later to Pauli about Weisskopf and the theory, he said, "Well, probably Weisskopf has done a very nice job; he carried out the mathematics quite competently," or something like that. Now Pauli is not a man who --. Of course, he was always very critical, and it was very difficult to hear from Pauli that somebody was really good. But I think he was right in that sense, that the essence of the theory he knew. This threw a different light, for me, on the Dirac theory. I think I understood what Pauli meant at that time, but I --. Well, he hadn't carried it out himself, but he evidently realized that if you want to have the one, you can't have the other one and vice versa. That was in 1929. Well, because I was rather depressed at that time, and also I lived at home with my parents again --. That didn't go very well any more because I was too independent and did not want to be the boy anymore at home. So I looked for the first opportunity to get away from Zurich. Indeed, I never lived in. Zurich again, after that. So in 1929 I had two possibilities: I could have gone to Born, who had an assistantship open for me in Gottingen; and Kramers also, who suggested, I think, to Pauli that he could get a fellowship for me in ... Utrecht. And I chose Utrecht -- I think that was partly also Pauli's influence, maybe also partly Heisenberg's. Both were somewhat critical of the Gottingen spirit; that is, Born's school and approach were considered highly formal and mathematical. I think it was this feeling with which I was inoculated which made me feel --. Well, I mean I had been in Gottingen before, and I knew people there, but I had the feeling, 'No, I'd rather go to Holland.' Besides, it may have been that I knew Germany already, and I wanted to go to Holland. So then in 1929 I came to Kramers, and, that was a very fine time because again he had a few students, but I think I was the first time that a Lorentz Fellow worked with him... And so he immediately accepted me as a friend, and. I was at his house, and he spoke to me about music and poems, of Oppenheimer, and things like that. But also physics, of course. And. I was very happy. The first thing I did at that time is that I caught this error in my thesis and wrote that paper. And then the second thing also: I found that at low temperatures Heisenberg's theory could be done better. Heisenberg had pulled some rather daring swindles there. I could show -- you know this business -- that when you go to very low temperatures, then almost all the spins are parallel --. I had essentially the idea of spin waves. And that gave an approach toward the treatment of ferromagnetism at low temperatures.

Kuhn:

I had meant to ask you earlier: In the published version of the thesis, the first of the conduction papers, you refer repeatedly to Hund's molecular work. You constantly are drawing comparisons with what Hund has said about the two-center problem, for example, and the symmetric and antisymmetric solutions. To what extent was that also a part of your research?

Bloch:

Now, glad you reminded me of it. Of course, Hund was also in Leipzig. Although my contact with Hund was not as close as that [with] Debye, of course I knew him well. Now, here I don't really quite remember -- I didn't even remember this particular point. But I suppose that it is true, that I started by saying, "Well, after all, one ought to generalize from two-center to many centers."

Kuhn:

One wouldn't necessarily do that. I mean this is an open question. There are to me a surprising number of references. One wouldn't have to take that approach, but one could.

Bloch:

I'm afraid I do not remember any more. I didn't even remember that I did refer to Hund's molecular orbits. It's quite possible --. Well, there was of course also a little tendency at that time in a thesis to show one's erudition. And that's, for example, why I did bring in group theory. One sort of had to show, "I know this." [Chuckle] "I know something, at least, about molecular orbits." As you say, I mentioned it, so it must have played some role in my thinking. I don't remember this too well. I suppose that must somehow have occupied me at that time, this idea, knowing after all very well the work of Heitler and London, from Zurich. But that was one approach to dealing with molecules, and then there was Hund's approach. And I somehow probably felt, I guess rightly, that Hund's approach was the more suitable one.

Kuhn:

Was the more suitable one for your problem?

Bloch:

For the conduction. problem, yes.

Kuhn:

Were you caught at all in the opposition between these two approaches?

Bloch:

Not seriously.

Kuhn:

This bothered some people, I gather, quite a lot.

Bloch:

Sure, of course. Now, I mean again, I had some sleep-walking qualities at that time, because of course I did realize that it was an over-simplification that one treated the one-body problem and then had many-body problems. But I felt, 'Well, that's probably all right. It works, so let's not worry about how good this approach is.' Later on, when people kept on using this same formalism and kept on refining it more and more, I was simply amazed because I thought, 'Oh, well, that's of course only an approximate theory.' In fact, I was already rather amazed that the resistance law came out as well as it did. I thought, 'Oh well, interactions can play a dirty role any old time.' And so I had a sort of sleep-walking -- I was not very critical, not towards myself, not towards other people. I just somehow felt, 'Well, these are the wave functions. They do the job, so let's stick to them.' And that they actually weren't much --. Well, there were other people who were surprised too, that it turned out to be such a good approximation. Well, so I think that essentially covers it up to 1930. I did go back to Leipzig in 1931 -- Wait a minute -- '28, '29, yes -- in 1930. In 1930 I was in Holland. I didn't spend all the time in Holland, by the way, with Kramers. I spent half a year with him, and then I was a few months or a few weeks -- very nice -- in Harlem. There was Fokker, of the Fokker-Planck equation. He was the director of a museum and laboratory there, Teyler's Stichting... When people go to Holland, they always go to the Frans Hals Museum. They should go to Teyler's Stichting, which is absolutely wonderful, both from the point of view of scientific instruments and art. And Fokker was the director. He wanted me as a sort of private tutor. He wanted to learn about Pauli matrices. He never had time, but (he) lived the life of a gentleman there. I had just a very nice time; I didn't really do very much. Then I went back to Leipzig in... the fall of '30. I stayed there until the summer of '31. There I was again interested in ferromagnetism, and I wrote a long and learned paper on ferromagnetism and hysteresis. I wrote a paper with [G] Gentile, an Italian, on ["Anisotropie d. Magnetisierg. ferromagnet. Einkristalle'], all down-to-earth problems, an extended paper which later on became my Habilitationsschrift. That I wrote in the summer of '31. What other things I was interested in at that time I. don't quite recall. I think Placzek was there, and maybe we talked about the Raman effect; I'm pretty sure we did. But nothing terribly profound.

Kuhn:

You had been away now since the very first year when you had been there before. There were many more students now. What did one feel the problems now were?

Bloch:

Teller was there then at that time. Teller worked with Hund on the ionized hydrogen molecule.

Kuhn:

Was the sense that physics had now solved its problems? Had one's sense of what there was for physicists to do changed drastically in this short time? Or was Leipzig just bigger but much the same?FB

Bloch:

I have a feeling that among the younger people talk went around that quantum mechanics was getting dull, that one was just doing some more of the applications, and that the most exciting things were really done. About the difficulties which were looming in the background, yes, I believe there was some talk about the beta decay and the difficulties of keeping electrons in the nucleus. I'm not quite sure whether it was at that time. But I think that was one of the things --. Again, I don't believe that I was terribly excited about this. There were difficulties, yes, but I did feel it was particularly my job to solve them. I didn't see any way of solving them, and probably I did not quite realize the profundity of it. I think that was only Bohr who convinced me that it was absolutely impossible to imagine an electron to be present in the nucleus.

Kuhn:

But your earlier attitude, you think, would have been more like your attitude towards superconductivity, that the model had eluded you so far, rather than that it was a point of fundamental importance?

Bloch:

A very good way of putting it, actually.

Kuhn:

Well now, you go from there to Bohr. How did you happen to do that?

Bloch:

I'd met Bohr before. I think it was Heisenberg's recommendation. Heisenberg thought that it would be good for me to spend some time in Copenhagen. Bohr invited me -- I don't know whether that came via Heisenberg -- and I could get an Orsted fellowship and go to Copenhagen, which I did at that time in a very devious way. I went via Russia; that was my first and last visit to Russia. I was invited. by Landau. I went east and went to Kharkov, Moscow, Leningrad, and then came through Finland and. Sweden. That's the way I came to Copenhagen. Then, well, it [Copenhagen] was a very strange thing. I tried to describe it somewhat in the Physics Today. Did you see that? Do you want a reprint of that, by the way? It gives a sort of feeling of the --. With Bohr one didn't talk about things in any systematic way, but Bohr liked very much to sort of think out loud. Since I was essentially--I lived in the same house -- he thought a lot out loud in my presence. He kept on going back to the indeterminacy relation, and what it all meant. He rather felt and gave me the impression that Heisenberg had rather oversimplified the problem. Really, it was at that time that I began to understand the whole problem of measurement, that one cannot show a sharp line of distinction between the observing subject and the object to be observed, and that this is really as profound as that, and that the whole principle of causality is at stake.

Kuhn:

Who talked about that in Copenhagen then except Bohr? By then I think nowhere except in Copenhagen were those problems still being discussed.

Bloch:

Of course, I was not everywhere, but I must say I know that in the places where I was, it was not discussed. Kramers was not particularly keen about these either. No, it was for the first time in Copenhagen that I felt they were important problems.

Kuhn:

I have some feeling that even some of the people around Bohr were no longer much interested in these.

Bloch:

We all, and I myself included, felt, very unjustly, that Bohr rather exaggerated these things. We felt, 'Yes well, they are important, they are illustrations already.' But Bohr kept on saying, "If somebody doesn't shiver when he learns quantum mechanics, then he hasn't understood it." It's a marvelous, marvelous --. But this, we felt, was rather --. We said, "We are not such cowards . We don't shiver." We realized of course what Bohr meant. And this dawned on me only very, very slowly, later, how profound this thing goes, and that one really has to shiver. Bohr said many things. Have you heard this wonderful statement -- he told that to me himself; he was rather proud of it -- that he made to Planck? Planck visited him early, maybe '26 or '27, and told him that he thought that quantum mechanics was so difficult to understand. Bohr said to him, "It isn't at all difficult to understand; it's impossible to understand." The causal approach that Planck wanted to bring into it. It was something that only gradually, I must say, and almost against my own wishes, became inculcated into me. It was Bohr's strong desire which --. Evidently he had a rather good opinion of me; he thought it was a shame that this man should run through life and be so blind and not even realize all the dangerous depths. He was rather insistent on that; and, as I say, I took it rather reluctantly because, as you may have gathered, I was more of the aggressive type and felt that solving problems was the interesting thing. Bohr said, yes, he was also interested, in problems, but this was the important thing. And he --

Kuhn:

He did this to you in walks together --?

Bloch:

In walks, or sitting --

Kuhn:

But it would be the two of you together usually?

Bloch:

Usually, usually. Sometimes it was also in seminars, which were very small. Muller was there. There was this poor man Solomon, Jacques Solomon, who was later shot by the Germans; the son-in-law of Langevin. He was there at that time. There were only a few of us. I think that I was not exceptional in that respect. Perhaps Muller, through longer acquaintance with Bohr, was more aware. Nevertheless, this idea that the old man is life too hard was rather general;tong us. Bohr was very kind, and had, a good understanding (for young people). Yes, these things I think mostly, I remember, came in talks. I was not supposed to. He was always behind in his publications and had to give speeches, and I promised him that I'd write them up. Then I was his assistant; he would discuss with me or occasionally I would jot a sentence down, which immediately had to be crossed out again. In the meantime Bohr walked 'round and 'round and 'round. Then he always said, "Let's talk about something else." And we talked about something else. So in bits and pieces, what I learned from him I got that way. He illustrated these things very beautifully. I don't think he invented that, but I do remember that it gave me a real jar -- I really understood for the first time how profound things are -- this famous double-slit experiment that you know. He published it later. That is to say, he said, "Really, you cannot possibly, you shouldn't be able to, tell whether the electron goes through one hole or the other, because, if you could tell, then you couldn't explain the facts." That gave me perhaps the first real shiver, but I said, "Yes, yes, it's true." I mean one has to abandon some very great prejudices.

Kuhn:

Has this had any real effect on your own later work?

Bloch:

Of course, as you know, I have never meddled myself with epistemology; it just doesn't suit my character. I do believe that it had some effect on it in the sense that I became more interested in the basic phenomena underlying some problem than just in producing results. I think it's rather clear in the paper which I wrote in Copenhagen. There I took up an old subject of Bohr, again through his stimulus, this question of stopping power of particles. I would not have treated such a problem before, because I felt that the answers were already essentially there. In classical mechanics, by Bohr; in quantum mechanics, by Bethe. Through the discussions with Bohr I felt there was something basically unexplained. I mentioned it to you briefly. I would say that a paper like that, which I said is essentially of pedagogical interest, I don't think I would have written before. I'm thinking back now of it with very great pleasure, and I'm very glad. I did it. But that was Bohr's influence.

Kuhn:

Did the greater sensitivity to that sort of problem stay with you?

Bloch:

Yes, I think that stayed with me. I think from that time on, whatever problem I dealt with, I've always felt more like saying, "Let's go back to the beginning; let's see what the foundations are," rather than to produce results. Although I turned away from engineering, I think I was too much of an engineer in regard to physics before I came to Bohr. Although I never became a philosopher, I think that besides producing results, basic understanding is an essential thing of physics really. It didn't become clear to me as essential until I met Bohr and talked to him about these things. There were many things I learned from Bohr. Not only about these but of course Bohr was a master in classical physics. And once in a while when he was in the mood, he was able to explain to you certain parts of classical physics in such an absolutely simple and marvelous way that I've never forgotten. In fact, you know, his own thesis was also on the conduction of electrons, so we had a common ground there. Once he told me, which I'd never known, in a very simple two lines you can derive, from a classical point of view, the expression for resistance. If you do the right sort of thing with the mean free path you get it in two lines. This is what Bohr was so great at. That is, he could get results within a factor of two or so. In order to understand the basic things you need not much mathematics that's what I learned from Bohr. Well, of course, this went on for a long time. Bohr made several statements at that time which I didn't appreciate and some of them I don't even appreciate to this day, and I feel bad about it. Bohr used to say and kept on saying that the dilemma in quantum mechanics is this: that all observations are essentially classical. That is to say, he said that the only way we can make contact with reality is through classical experiments. And this is where all the difficulty arises, you see. This is not --. I don't think I want to go into it because in some sense it is clear; in another sense, why it should be quite as important as that --. I must say, it's only maybe in the last ten years or so that I have a certain feeling for that. But I'm not this type, you see. I've not gone deeply into that problem, and Bohr felt very sorry about it and probably felt it was one of my shortcomings. I think he felt one should go into these questions deeply.

Kuhn:

Of course he did. It's, I think, some part of the tragedy of his later life that no physicists really followed him in pursuing that concern.

Bloch:

Of course, one of the reasons was that probably we all felt, "There's no use going into that because Bohr is so far ahead of us anyhow that one can only play a second-hand role." I mean Rosenfeld, rather touchingly, did that. Rosenfeld was really just a help to Bohr. Bohr wanted somebody who knew mathematics and was willing to go through it; [Rosenfeld] sort of sacrificed himself. But it's true, I also have a feeling that --. Some of his remarks I remember; I'm sure many I have forgotten. If I did remember them and other people remembered them and would think more about them, we would probably see, well, in some cases he was wrong. For example, the question of beta decay. He was quite convinced that energy was not conserved. Well, all right. It was not that he was always right.

Kuhn:

No. That particular idea -- that energy is not conserved -- is one that he had over and over and over again in his career.

Bloch:

Yes, yes. He never felt that the conservation of energy is something so very, very sacred. That was, of course, more a formal argument, but Pauli --. The reason why Pauli didn't accept that was that Pauli, I think, appreciated more than Bohr, from a formal point of view, that "How could one do all these things if one didn't have a Hamiltonian?" And of course, the moment you have a Hamiltonian, you do have energy conservation. Pauli just thought, "Goodness! If we give up energy conservation, what a mess we will be in from a mathematical point of view;" Bohr was never impressed by such things, you know; he always felt, "The mathematicians will take care of that." I'm quite sure that it was this kind of thing that made Pauli invent the neutrino. He just felt, "I'd rather buy a new particle than make my life that complicated." Bohr did speak to me occasionally about the early days, and he gave me -- in fact I have them still here -- some of the reprints of the early years, 1916 and so on. He pointed out to me, and quite rightly, that he did not take his own model as seriously as many other people did. He was very proud of that. The strange thing was that Sommerfeld who, of course, at first was skeptical towards Bohr's theory, later on took it much too seriously, or at least Bohr felt so. He felt really that quantum mechanics was the answer to what he was always looking for. About these things he spoke to me. But that of course you know.

Kuhn:

There are things that he said to you that I'd like to know but don't know, but in general we have been over much of that ground. There are parts of it we can just not know, partly because Bohr himself died too soon in our work with him. The whole period that Kramers could have filled in --

Bloch:

But Oscar Klein must have known a lot about it. Klein and Kramers were there at about the same time, think.

Kuhn:

No, Kramers was really two years earlier than Klein and was sufficiently further along early, so that there are key developments in the whole idea of the correspondence principle in this period before Klein is involved, and even more before Klein is involved with real understanding of what is going on.

Bloch:

This was before the dispersion formula? Kramers never talked with me about these times. I'm not sure; I don't remember it.

Kuhn:

You go from Copenhagen in '31 back for '31-'32 again in Leipzig?

Bloch:

I came to Copenhagen in the fall of '31; I went back to Leipzig in the spring of '32. There I was habilitiert. (I became a) Privatdozent.

Kuhn:

Right, and then you're there '32-'33. This is just the period that the neutron, the positron, the Heisenberg theory -- these and generally this whole question of quantum mechanics in the nucleus became very --.

Bloch:

There I have a very deliberate impression. One day -- oh, it was in the late afternoon -- Heisenberg came by to the house where I lived, which was not very far away from the Institute, and said, "Oh, I'm glad you are here. Would you come with me on a walk? I'd like to tell you something." And on that walk he said, "I just want to have your reaction. I just thought about nuclei." Now the neutron was discovered at that time.

Kuhn:

Do you remember getting this news? Was it a big shock to people?

Bloch:

Yes, oh yes. In fact I think Bohr told me that. You know, after I was in Copenhagen, I met Bohr frequently. On one occasion we met in Berlin, and I think he told me in Berlin about it, about the discovery of the neutron. Oh, yes, he was very excited; so were we all. This was maybe -- oh, I'd say that was perhaps not even a year later. am pretty sure that it was in the ... early summer of '32 or so. Heisenberg took me on a walk and told me about this idea. I was simply amazed. Maybe I said that I was surprised, but mostly I listened. I remember I thought, "How can Heisenberg know all these things (for so sure)?" He said, -- and this I was willing to accept -- "Of course, neutrons are --." This business of the electron -- I've told you before about that difficulty (of the nucleus containing the electron.)" I said to Heisenberg [Heisenberg said to me], "This is fine, and everything will come into order. Neutrons have, of course, a spin, and their mass is equal to that of the proton, or practically equal. Then all these questions of statistics will get into order, right?" And so he said--

Kuhn:

He saw that immediately, that statistics would get straightened out?

Bloch:

He mentioned that so casually that he made it sound trivial. There, I must say, to me it didn't sound trivial at all. In fact, as I say, I was surprised at Heisenberg's --. But he went directly on. He just said, "Now, you see, this is rather interesting, because why is it that the atomic weight is almost exactly twice the atomic number?" "Well," he said, "this is a symmetry. There must be a symmetry." And in fact he had already a sort of a rough mass formula--I think it was later called the Weizsacker mass formula--but I think it was more the Heisenberg mass formula, where he pointed out that there must be for symmetry reasons a minimum in the PZ diagram, and that it was shifting the other way because of the Coulomb forces, for the heavier ones. So he had all the essentials there. He was very excited about it, and I just didn't say very much. He really just wanted to see whether, by any chance, I could catch him on a gross mistake. I think I asked him occasionally, "Are you sure of that?" He said -- well, Heisenberg was very cavalier in such things -- he said, "Well, that's probably --" There was one more thing that Heisenberg mentioned at that time which amazed me, and there I was even more impressed. That is to say, he had the idea that the distinction -- that the proton and the neutron were really two states of the same thing. He didn't use the word "nucleon," to be sure, but he had the idea of a quantum number. I think later on it was called the isotopic spin. But he said to me that they were clearly two quantum states of the same particle. In an outline, I got the nuclear theory from Heisenberg on a walk, and he published it immediately afterwards. He was very quick in writing such things up. There was a little thing about --. Shortly afterwards I was then also somewhat interested in the neutron, and in fact I was beginning to get interested in nuclear magnetic moments. I had an idea which Heisenberg accepted. It was the wrong reason, but there was a little grain of truth in it. I must have accepted right away that the neutron has a spin, and then I knew that even nuclei -- even atomic nuclei -- did not have a magnetic moment. Of course, this is nonsense because it's because their spin's zero. But I had the idea that this is an exact compensation of the positive [magnetic moments of the protons] and the negative magnetic moments of the neutrons, because they are in equal numbers, you see. I suggested that to Heisenberg and said to him -- I don't think I published it anywhere; no, I don't think I did -- but I said to him, "I think the neutron ought to have a negative magnetic moment equal and opposite to that of the proton." Heisenberg liked that idea. You can see what a fantastic instinct this man had, because of course the conclusion wasn't so wrong, but the reason was completely wrong. Then, of course, shortly afterwards -- that was also before I left Leipzig -- Stern talked at one of the colloquia about his measurements of the magnetic moments of the proton and the deuteron. Then we knew. I mean we knew indeed it had a negative [magnetic moment] but not exactly equal. Then things became clearer. But that was the only direct effect that Heisenberg's idea had, on me at that time -- in connection with the magnetic moment. Somehow I've always spun the same thread in my whole life, as you see, because it was always magnetism. As soon as neutrons and protons were mentioned, I asked what were the magnetic properties. That was something which somehow suited my character.

Kuhn:

What about the positron, which comes in very much at this same time, but which is not so expected?

Bloch:

That I don't remember. I'm not quite sure whether I didn't learn about the positron only when I came here.

Kuhn:

That's late.

Bloch:

Well, it was '34.

Kuhn:

No, you must have had it in Rome; the positron is also '32. This really comes very little after the neutron.

Bloch:

Pair-production, yes. I don't know. In Rome there was one thing, since you asked me about quantized amplitudes. his is the sort of game I played in Rome. I had written another paper on the energy -- it had to do with stopping power too -- and there I had considered the question of the vibration of a gas sphere. I published that. That was later; it was in '32, I believe. When I was in Rome, I wanted to understand a little bit better how oscillations -- plasma oscillations and oscillations of electron gases -- could be explained in terms of the elementary mechanism. I thought at that time that the quantized amplitudes might be a good handle to do it, because you had, so to say, waves. The quantized amplitudes had wave character. Well, it was sort of a vague idea. Interestingly enough, there was not much contact with Fermi. We had personal contact with him -- played tennis and told each other jokes, but very rarely talked physics. Fermi didn't really like very much to discuss physics. But at one point I just felt I wanted to --.

Kuhn:

Elaborate that remark that he didn't like to --. Do you mean that he didn't like to talk theoretical physics?

Bloch:

He didn't like to solve other people's problems, or to be bothered by other people's problems. He wanted to do his own problems. If people came and tried to push him into a line of thinking in which he was not interested at that time, he didn't like that particularly. I mean he was not impolite, but he made it rather plain that he wasn't too much interested. And that time I sort of crashed his doors -- that was the only time I did it -- because I felt there was something vaguely resembling it, and I started to tell him about the quantized amplitudes. Fermi said to me in Italian, "I don't understand a single word." "Non ci capisco una parola," he said. It is true. He did not understand quantized amplitudes at that time. He just simply never had studied it. I'm sure he did not --. The problem which I was worried about didn't interest him at all. But I think he made a mental note at that time, "Perhaps I ought to look at quantized amplitudes." And he wrote his neutrino paper a few months later. This was typical Fermi. You see, he didn't think, as I did indeed before too, that quantized amplitudes were of much use. When he'd explain things in the seminar or so, he always went the very pedestrian way -- x1, x2, x3 -- he always wrote them all down. And then he thought, "Well, perhaps I ought to learn that." When Fermi learned something, he didn't just learn it, but he used it. Well, of course he realized the relation between quantized electron amplitudes and quantized field amplitudes, and the possibility of emission of electrons, of creation. I think actually I can really say that Fermi did not know much about quantized amplitudes in the fall of 1933.

Kuhn:

That is very interesting, because it then comes so quickly; and it's exactly at such a strategic time that he chooses to learn about them.

Bloch:

I'm quite sure that for him the beta decay was sort of a school example, one of the nice applications of quantized amplitudes.

Kuhn:

Very nice --

Bloch:

Very nice. The positive electron -- Gosh, I wish I could remember when that came first to my attention. You say that was '32?

Kuhn:

Yes. I can't remember when, but '32. There, you see, my memory is not so very good. I have the impression that I heard about it later, but I must have heard about it earlier.

Kuhn:

I asked because the whole reaction to the neutron and to the positron was very different in some places. Bohr, for example -- it was terribly hard to persuade Bohr that there was such a thing as the positron. me, how did you happen to come to Stanford? How you came to America think the time makes fairly clear. But how did it happen to be Stanford?

Bloch:

In detail I do not know. The thing was this -- well of course, it was known that I had left Germany because of Hitler, together with several other people -- Franck and Einstein and so forth. At that time the Rockefeller Foundation offered to various universities funds to take on these people. They would pay, I think, for a year or two, their salaries. They would do that providing that these institutions had at least a serious intention of keeping the people if they were suitable. Why it was Stanford --? I think there must have been several reasons. In the first place, since this work on fast-moving particles was of some interest to the people here who were working in X-rays at that time, they knew about this work -- [D. L.] Webster did, I think; and I think Oppenheimer has at one point indicated to me, that Oppenheimer had also something to do with it, because I knew him from Zurich before. I think this was the reason why I got an invitation to come to Stanford. That was when I was visiting in Copenhagen in '33. Needless to say, I accepted it. That's all. Actually ... I had a Rockefeller fellowship at that time. When I applied for [it], I said I wanted to spend half a year in Rome and half a year in Cambridge. Then I cancelled the Cambridge thing; I said I'd rather go to America. That's how I came here.